I. Field of the Invention
The present invention relates to an electrochromic display device and, more particularly, to an electrochromic display device using an organic electrochromic material.
II. Description of the Prior Art
Conventionally, an electrochromic (EC) display device includes a display electrode comprising an EC layer on a transparent conductor film and an opposing electrode, and has a structure wherein a liquid- or solid-phase electrolyte layer is sandwiched between the two electrodes. Both inorganic and organic EC materials are used.
Transition metal oxides such as WO.sub.3 and MoO.sub.3 are known as inorganic EC materials. An inorganic EC material is deposited on a transparent conductor film. When a display electrode of the EC display device using WO.sub.3 is negatively biased, electrons are injected into the WO.sub.3, so that the display electrode becomes blue. In the device of this type, a material having stable reversible potential must be used as an electrode opposing the display electrode. In general, a symmetrical cell using reduced WO.sub.3 is used. However, the reduced state changes during operation of the device, thus resulting in an unstable display state and low reliability.
A viologen compound, a pyridine compound and a rhodamine compound are known as organic EC materials. A typical example is a device wherein viologen is sandwiched between a pair of electrodes. When the display electrode is negatively biased, viologen is one-electron reduced and is precipitated on the display electrode, so that the display electrode is colored. However, in the device of this type, the anion radical changes over time after precipitation. As a result, the color will not disappear even when the reverse bias voltage is applied, resulting in inconvenience.
As described above, the reversibility of the conventional EC display device is not complete, and the display element has a short life and poor reliability.